Shredder knives wear off quickly

Hi all!

We’ve recently built a new shredder machine and have all in all shredded no more than one 1kg of plastic. However, the tips of the shredder knives seems to wear off. They get twisted or bent in a kind of bad way. I’ve posted some photos to show this.

Any ideas, why this happens and whether this is expected or not? Any way to fix this? I was thinking of dismantling the machine and sharpen them again but in a way so that the tips or thicker.

Btw, we’ve used the typical eisa304 stainless steel quality suggested in the manual.



My initial thought would be that they were catching on the mesh. perhaps spacing was off a little bit on the build? They appear to be rolled over and not chipped, which would make me think they hit something hard. could there have been metal contamination in the plastic your ground? pop cans or something?

thanks for the reply, in case we need to have a good shredder

I saw your description of the problem, I guess, whether the plastic you crushed contains metal impurities. If it is pure plastic, the blade wear will not be so fast; there is also whether the speed of your feed is a bit fast or other faults during the operation of the machine cause high temperature. Therefore, my suggestion is that you will consider cutting the blade or replacing other types of blades after eliminating these factors.

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this topic also discuss about “wear off”

Thanks @justinc. It’s good to have sth to compare with.

Here are photos of the blade preparation, both tips and shearing edges should be sharpened to improve efficiency and durability. (Modified parts on bottom)

I have got a request for this problem: Has anyone got a pic with propper sharpend blades? My schredder is about to grow up, and i want it to work propper and long.


Silicon steel like leaf springs one is a great suggestion too because it’s hard and elastic…

C40 is the cheapest option, hardenable and about 1€/kg…

AISI 304 is good for not taking rust, but soft and more expensive than a carbon steel…
Here hints on how to harden steel: How to Harden Steel: 10 Steps (with Pictures) - wikiHow
hardenable steel:

Thanks for the suggestions and feedback everybody!


When you say you ‘grind the tips’, what would you suggest as an angle of the cutting edge of the tip. I figure that you refer to the tip of cutter (the one that seems deformed in the first posted images) and not the sides. right ?


On (1), yeah, i fear that having blunt edges will most probably make things worse. Again can you suggest an angle for the tips after grinding ? Or thickness ?
On (2), unfortunately our level of expertise and the access to resources does not allow changing the type of steel at present. But yes, from my initial investigation AISI304 was not the best type of steel to use. At least in terms of hardness.
On (3), no. We didn’t use any reinforces plastics. The hardest thing we shredded was the necks and bottoms of PET bottles.


Will keep that in mind. Congrats on re-using the ‘good stuff’ 😉




with me having some experience with machinery I see 3 items that could cause the Problems mentioned.

1. As already mentioned cutting Tools should be sharp. If they are dull using them often destroys them completely. If the cutting edge turns over maybe the cutting angle is too small. Some experimentation with larger (less Slim) edges may be worthwhile.

2. I don’t think material selection is very successful here. AISA 304 steel is similar to ISO 1.4301. It’s an austenitic stainless steel, ductile but not really hard and with a tendency to gall very fast. And it can’t be hardened by heat Treatment.
Use of a tool steel might work better, a cold working tool steel should do best.
In ISO theres for example silversteel 1.2210 or even better 1.2842 will do better.

3. You might have cut reinforced Plastics. A common plastic for high strength applications is for example PA6 + GF30% .
Ist Polyamide with 30% glass fibre as reinforcement and will dull Tools pretty quickly.
I’ve milled or turned a lot of plastic with HSS and usually the cutter will remain sharp for hours, even 2 or 3 days. But with glass fibre inside, the cutting edge is done in an hour or less and you Need carbide tooling.


The tips of the blades should be ground to chisel points which are much stronger and cut better than the thin tips on the laser cut parts.  I always grind the tips off and re-grind with an angled chisel tip.  It is extra work but worth the effort.

bastelmike has a point. there are other options out there regarding materials for the knives, maybe better than AISI 304.
Here in Cabo Verde is dificult to find special materials like tool steel or other high carbon content metals…we have to import everything.
Over the years we have learned to adapt and use materials available in other apllications. For glass crushers and manual plate/sheet shearing machines for example, we use spring steel (AISI 6150), found in leaf spring on auto suspension. The material is curved but you can heat them up a litlle bit and straight them out. You have cars everywhere so plenty of “good stuff” to work with.


No problem, and good luck on your recycling!

Great, thanks for the valuable feedback Rory!

Yes exactly, a blunt blade still cuts but requires more pressure and is more likely to deform. I personally haven’t had the chance to test myself. My shredder is hand powered and is currently being modified.

Thanks for the response Rory. What you suggest makes sense to me.

So, your rational if i got this right is that the tips of the blades should be sharp. In my case they are not (and probably weren’t from the start). So they couldn’t  easily cut through the plastic and were pushed against it instead. That, resulted in the deformation of them and the quick wear.

Hey @otsakir,
We also have small signs of this occurring. Have you tried sharpening the tips of your blades with a file yet? It may solve your problem.